1. Field of the Invention
The invention relates to a method of assigning binary values to a series of sample values taken periodically from a waveform representing a binary pattern of marks on a record carrier, the pattern of marks nominally having a minimum run length of more than one sampling period. The invention further relates to detector apparatus for performing such a method and to an optical disc player including such a detector.
2. Description of the Related Art
The method described can be referred to as "bit detection" and may be used in particular in reading information from optical record carriers. A well-known example of such method is in the Compact Disc (CD) system for audio and data recording. The CD and other optical recording systems have been described in "Principles of Optical Disc Systems" by G. Bouwhuis et al, published by Adam Hilger, Bristol, UK (ISBN 0-85274-785-3). In the CD system, for example, a minimum run length of three samples is assured by virture of the Eight-to-Fourteen Modulation (EFM) employed. Many other kinds of minimum run length modulation are also known, however.
Disturbances may be introduced by the reading apparatus (CD-player) into the information read from a record carrier, for example because of optical effects during reading. It is known from the above reference that such disturbances may be regarded as linear and (quasi-) stationary to a good approximation. In the event of such disturbances, and if noise plays a subordinate part (as it does in the CD-player), the recorded data sequence can be recovered reliably by means of the customary combination of a fixed linear equaliser and a threshold detector. A threshold detector for use in optical recording apparatus is disclosed for example in U.S. Pat. No. 4,499,570 assigned to the present assignee.
Properly manufactured CD records do not introduce any significant additional disturbances. However, it is found that in practice there are discs which have pits which are systematically too short or too long, in particular as a result of imperfect etching in the mastering process. In extreme cases this may result in the minimum run length of zeros being increased or reduced by almost two sample periods relative to the nominal EFM value of 3 sample periods, and similarly for the minimum run length of ones. Because of the non-stationary (or, depending on the point of view, even non-linear) nature of this imperfection a fixed (and consequently time-invariant) linear equaliser is intrinsically not capable of effectively suppressing this disturbance.
It is customary therefore to make a compromise and design the equaliser such that even in extreme situations a certain decision margin is still left for the threshold detector. However, since this margin is small, even minor disturbances in, for example, the speed of rotation, or in focussing and clock generation, may lead to decision errors. Correction of these errors requires part of the capacity of the error-correction system of the CD-player, which is then no longer available for reducing the effect of, for example, scratches and finger marks on the record carrier.
As well as the case of poorly-manufactured Compact Disc records, systematic error is also a problem in the field of writable optical recording systems, where recordings are made on a large number of individual recorders rather than in a carefully monitored factory environment.